1. Field of the Invention
This invention relates to the use of ink jet printing for metal-image formation in an improved process for making printed circuit boards.
2. State of the Art
Ink jet printing is a nonimpact technique for projecting droplets of ink onto a substrate. One typical method employed for standard printing of, say, a page of alphanumeric characters is as follows. A conducting ink is supplied under pressure to an ink gun and forced out through a small nozzle of typically 35 to 80 .mu.m diameter. As it passes through the nozzle the liquid is piezoelectrically pulsed (or modulated) and the stream breaks up into a continuous series of drops which are equally spaced and of equal size.
Surrounding the jet at the point where the drops separate from the liquid stream in a charge electrode. A voltage is applied between the charge electrode and the drop stream and when a drop breaks off from the stream it therefore carries a charge proportional to the applied voltage at the instant at which it breaks off. By varying the charge electrode voltages at the same rate as drops are produced it is therefore possible to charge every drop to a predetermined level. The drop stream continues its flight and passes between two deflector plates which are maintained at a constant potential, typically .+-.2.5 kV. In the presence of this field a drop is deflected towards one of the plates by an amount proportional to the charge carried. Drops which are uncharged are undeflected and collected by a gutter to be recycled into the ink gun. Those drops which are charged, and hence deflected, impinge on a substrate traveling at high speed at right angles to the direction of drop deflection. By varying the charge on individual drops, the desired pattern can be printed.
The ink jet process is adaptable to computer control for high speed printing of continuously variable data. Ink jet printing methods can be divided into three general categories: high pressure, low pressure, and vacuum techniques. All have been described and employed in conventional ink jet printing and can be employed in the process of this invention.
Reviews of various aspects of conventional ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, pages 162 to 178; and Keeling, Phys. Technol., 12(5), 196 to 203 (1981). Various ink jet apparatuses are known and can be used in the process of this invention. Typical apparatuses are described in U.S. Pat. No. 3,060,429, U.S. Pat. No. 3,298,030, U.S. Pat. No. 3,373,437, U.S. Pat. No. 3,416,153 and U.S. Pat. No. 3,673,601.
German Patent Specification No. 3,047,884 discloses the preparation of printed circuit boards employing an ink jet printer. If the recording head were heated, metals in liquid form could be sprayed. Also disclosed is the spraying of organometallic solutions such as organocopper compounds directly on the unmetallized circuit board. By means of a subsequent laser beam the conductor pathways can be cured completely.
Vest et al., in Intl. J. Hybrid Microelectronics 6, 261 to 267 (1983), disclose computer controlled ink jet printing of hybrid microelectronic circuits. A ink-jet printer is used with a conductor ink based on metallo-organic compounds in solution. The use of silver neodecanoate as a silver conductor is disclosed, with or without added platinum in the form of platinum amine octoate, to produce a solderable connector. Silver conductor line patterns on glass and alumina substrates were produced, the silver inks decomposed to silver when heated to 250.degree. C.
Japanese Kokai No. 66089/1981 discloses a printed circuit board obtained by ink-jet printing a copper foil on a plastic substrate with an acid-proof ink containing a material such as silicon varnish which solidifies on drying, and etching the undesired portion of the copper foil.
British Patent Specification No. 1,431,462 discloses production of relief images using an ink jet method to modify the solubility properties of a polymer layer such as gelatin by spraying a solution of an appropriate material such as formaldehyde in conformity with the image areas.
Concerning electroless plating and conventional techniques for plating printed circuit boards, see: Coombs, Jr., Printed Circuits Handbook, Second Edition, McGraw Hill, New York City (1979), pages 7-1 to 7-9, 22-12 and 22-13; and Horkans et al., IBM J. Res. Develop. (1984) Volume 28, pages 690 to 696.